Cylindrical shells are typically used for encasing motors, generators, and other rotating dynamoelectric devices, such as fractional horsepower motors. In the prior art, these cylindrical shells have been formed by a process which involves first cutting a flat rectangular piece of sheet steel, rolling it into a cylinder with the edges abutting, and resistance welding the edges together. The resistance weld may be cleaned up by grinding or the like to finish the shell cylinder The diameter and length of the shell is then checked and adjusted to size. Next, a rigid base for mounting of the motor is attached to the shell by another welding process at what may be six locations in the cradle of the base. This process is widely used throughout the motor industry for forming cylindrical shells and attaching bases to those shells to encase motors
In order to improve this process by reducing steps and costs, the inventor herein has succeeded in designing and developing a process for forming a cylindrical motor shell, as well as attaching a base thereto, which eliminates welding, grinding, and permits production line processes to be used. With this invention, a process is implemented which includes the steps of forming a blank of a flat rectangular piece of sheet steel, as before. Next, an edge of the blank is offset so that as the blank is rolled into a cylinder, the offset edge conveniently overlaps with its opposite edge. The overlapping edges are then joined with a single press stroke which interlocks the edges. The tooling and interlocking method is commercially available under the trade name Tog-L-Loc.RTM. from BTM Corporation in Marysville, Mich. With the interlock formed by the Tog-L-Loc.RTM. process, not only are the overlapping edges joined to withstand significant shear loads, but may also withstand "peel" loads which might otherwise tend to separate the shell as the motor is used under load. This is especially important as the Tog-L-Loc.RTM. method is also used to attach the base to the cylindrical shell either simultaneously with joining the overlapping edges, or in a separate step after the cylinder has been formed. As can be appreciated, with a side load of any appreciable amount, a force is created which has a tendency to "peel" the motor and shell from the base. With the Tog-L-Loc.RTM. interlock, sufficient strength is provided to prevent this "peeling" and hence is sufficient for this application.
As is well known, there are many punching, staking, and other prior art methods for joining two pieces of metal which, to the inventor's knowledge, have never been successfully used for this application. All of these prior art methods are directed to providing shear strength and are generally considered by those of ordinary skill in the art to adequately join two pieces of metal to resist shear forces. However, the same is not true for peel forces and it is believed that this low peel strength has prevented the adoption and use of these prior art punching, staking methods for being utilized for this particular application. Also, for smaller motor shells as would be used in fractional horsepower motors, those of ordinary skill in the art would typically consider that prior art tooling would seem to be too expensive, or very difficult, to create.
Perhaps another reason why those of ordinary skill in the art have been unsuccessful in adapting any kind of punching or staking operation to this application is that the typical motor/generator design incorporates a full round lamination. Therefore, if the inner surface of the cylindrical shell is interrupted, then a full round lamination will not conveniently fit therewithin. However, there are other motor/generator designs which do not utilize the full round lamination and hence would be particularly adapted for use with the present invention.
In addition to the method and apparatus for forming and joining the cylindrical shell to its base with an interlocking process, the inventor herein has also succeeded in developing a method useful in this process for holding the cylindrical shell I.D. as the interlocks are formed without the need to "size" or expand the shell to proper dimension after forming, as is presently done with the prior art processes.
While the principal advantages and features of the present invention have been described above, a more complete and thorough understanding of the invention may be attained by referring to the drawings and description of the preferred embodiment which follow.